Improving the delivery of drugs and other agents to target cells and tissues has been the focus of considerable research for many years. Though many attempts have been made to develop effective methods for importing biologically active molecules into cells, both in vivo and in vitro, none has proved to be entirely satisfactory. Optimizing the association of the inhibitory drug with its intracellular target, while minimizing intercellular redistribution of the drug, e.g., to neighboring cells, is often difficult or inefficient.
Most agents currently administered to a patient parenterally are not targeted, resulting in systemic delivery of the agent to cells and tissues of the body where it is unnecessary, and often undesirable. This may result in adverse drug side effects, and often limits the dose of a drug (e.g., glucocorticoids and other anti-inflammatory drugs) that can be administered. By comparison, although oral administration of drugs is generally recognized as a convenient and economical method of administration, oral administration can result in either (a) uptake of the drug through the cellular and tissue barriers, e.g., blood/brain, epithelial, cell membrane, resulting in undesirable systemic distribution, or (b) temporary residence of the drug within the gastrointestinal tract. Accordingly, a major goal has been to develop methods for specifically targeting agents to cells and tissues. Benefits of such treatment includes avoiding the general physiological effects of inappropriate delivery of such agents to other cells and tissues, such as uninfected cells.
An antimetabolite is a compound that interferes with, e.g., competes with, replaces, or antagonizes, the utilization of a particular metabolite. An antimetabolite can interfere by means of having a similar chemical structure with the metabolite, i.e., the antimetabolite can be an analog, e.g., a nucleoside analog, of the metabolite. Antimetabolites work through a variety of mechanisms and can, e.g., interfere with the production of nucleic acids. There are several categories of antimetabolites including antifolates, purine analogs and pyrimidine antimetabolites. The nucleic acid and folate antimetabolites can act by inhibiting DNA and/or RNA synthesis and can have their most toxic effects on rapidly proliferating cells. Because of these and other properties, antimetabolites are used to treat, e.g., cancer and viral infections.
Several antimetabolites have been identified as useful therapeutic compounds. For example, Raltitrexed (U.S. Pat. No. 4,992,550), Aminopterin (U.S. Pat. No. 2,575,168), Pemetrexed (U.S. Pat. No. 5,344,932), and 10-Propargyl-10-deaza-aminopterin (PDX) (U.S. Pat. No. 6,028,071) have been identified as a useful therapeutic agent for the treatment of diseases such as leukemia and lymphoma. Thier antiproliferative effects are also applicable in a variety of other disorders associated with inflammatory or immunological phenomena such as psoriasis or rheumatoid arthritis.
Additionally, Methotrexate is an anti-neoplastic agent and has been shown to be useful against rheumatoid arthritis. Methotrexate functions as an antimetabolite by depleting the folate pool inside cells through inhibition of dihydrofolate reductase (DHFR). Side effects include mouth sores, diarrhea, fever, persistent sore throat, unusual bleeding or bruising, black stools, rash, enlarged glands/lymph nodes, bone pain, unusual pain and skin discoloration, stomach pain or dark urine. Methotrexate is metabolized to a polyglutamate derivative that is active as an inhibitor of DHFR.
Thus, there is a need for therapeutic agents that are antimetabolites with improved pharmacological properties, e.g., drugs having improved antimetabolite activity and pharmacokinetic properties, including improved oral bioavailability, greater potency and extended effective half-life in vivo. Such antimetabolites would have therapeutic potential as anti-inflammatory agents, immunosuppressants, antiviral agents, and as anti-cancer agents. Specifically, such compounds may be used in the treatment of viral infections, cancer and inflammatory and immunological phenomena such as psoriasis or rheumatoid arthritis. New antimetabolites should have fewer side effects, less complicated dosing schedules, and be orally active. In particular, there is a need for a less onerous dosage regimen, such as one pill, once per day.
Assay methods capable of determining the presence, absence or amounts of antimetabolite activity are of practical utility in the search for antimetabolites as well as for diagnosing the presence of conditions associated with the metabolites and/or antimetabolites.